The present application is based on Japanese Patent Application No. 2002-143335, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention is related to an optical switch employed in, for instance, optical communication systems. More specifically, the present invention is directed to a waveguide type liquid crystal optical switch for controlling switching operations of optical paths between waveguides by way of liquid crystal.
2. Related Art
Since large amounts of information can be transmitted/received in optical communications, very recently, these optical communications are gradually popularized in many homes. These optical communications correspond to transmission systems by employing optical fibers. In order to distribute optical signals to respective terminals of such transmission systems, various sorts of optical components known as fiber type optical couplers, optical wavelength division multiplexers/demultiplexers optical wave guide type branching/combining devices, and optical switches have been developed.
Among these optical components, optical switches may have switching functions of optical paths, and thus, may become important as switching devices for optical communications. Conventionally, various sorts of optical switches utilized in optical communications are known in this technical field. Among these optical switches, since such an optical switching system that while optical waveguides are used, optical transmission paths are switched by utilizing various sorts of physical phenomena is not equipped with any mechanical movable portion, this optical switching system has merits of high reliability and highspeed operations. As such an optical switching system, for instance, various optical switches are known. That is, there are optical switches using dielectric crystal waveguides such as LiNbO3 having either the electrooptic effect or the acostooptic effect; optical switches using semiconductor waveguides, while carrier injection is utilized; and optical switches using silica-system waveguides, while the thermooptic effect is utilized.
Also, among optical switches equipped with similar optical waveguides, optical switches using liquid crystal are known. Liquid crystal may have the electrooptic effect under which refractive indexes are changed by applying electric field to the liquid crystal, in view of abroad sense Also, liquid crystal may have various characteristics, namely, liquid crystal may be driven under low voltages, may have high reliability (actually realized as display devices), may have high productivity, and may be manufactured in low cost. As such optical wavelength type liquid crystal switches, for example, Japanese Unexamined Patent Publication No. H05-165068A describes such a waveguide type liquid crystal optical switch constituted in such a manner that two parallel single-mode optical core patterns are formed on the lower cladding and are equipped with the coupling portions, the partial portions of which are located in close proximity to each other; and furthermore, the lower electrode is formed on the lower cladding of the coupling portions, the coupling portions are filled with the orientated liquid crystal; and the coupling portions are sealed by the glass plate having the upper electrode.
However, in this waveguide type liquid crystal optical switch, since both the liquid crystal and the electrodes are formed in such a manner that the substantially entire portion of the lower cladding is covered, the refractive indexes are changed over the wide range of the lower cladding, so that the guided mode may be disturbed, and large crosstalks happen to occur. Such large crosstalks may cause a fatal defect with respect to optical switches. Moreover, since the three planes of the coupling portion of the waveguide core are made in contact to the liquid crystal, if there are such liquid crystal molecules which are not orientated along the predetermined direction, then the scattering loss caused by these not-orientated liquid crystal molecules is accordingly increased. Also, the lower electrode is located very close to the coupling portion of the waveguide, so that the loss caused by this lower electrode becomes very large. In addition, since this waveguide type liquid crystal optical switch has such a construction that only one pair of upper/lower electrodes are limitedly arranged, this optical switch has another problem that the polarized-wave dependent characteristics thereof are strongly emphasized.
The present invention has been made to solve these problems of the conventional optical switch, and therefore, has an object to provide a waveguide type liquid crystal optical switch capable of improving crosstalk performance and insertion loss and further having no polarized-wave dependent characteristic in addition to tho merits specific to such a waveguide type liquid crystal optical switch, namely, low cost, high reliability, and low power consumption.
To achieve the above-described object, a waveguide type liquid crystal optical switch, according to the present invention, is featured by that in an optical switch equipped with an optical waveguide having one pair of a first core and a second core located in proximity to the first core, for entering light into any one of the first core and the second core and for switching an optical path of the entered light between both the first core and the second core, such a waveguide type liquid crystal optical switch is comprised of: a third core provided apart from both the first core and the second core in such a manner that the third core covers a space sandwiched by both the first core and the second core, into which nematic liquid crystal orientated by an orientation film along a predetermined direction is filled; a first electrode of the third core, arranged on the opposite side with respect to both the first core and the second core in such a manner that a gap portion between the first core and the second core is covered by the first electrode; a second electrode and a third electrode, which are arranged to constitute a pair by sandwiching the first electrode, and cause liquid crystal molecules to be orientated along a direction perpendicular to the orientation direction of the orientation film; and a cladding for incorporating the first core, the second core, the third core, and the first to third electrodes.